B737-300 WAS PARKED AT GATE AND ENGS SHUT DOWN. ACFT BEGAN ROLLING BACKWARD AS A RESULT OF CHOCKS NOT INSTALLED PER COMPANY SOP.

Narrative

AFTER AN UNEVENTFUL FLT; WE TAXIED TO THE GATE. WE SHUT DOWN ENG #2 PRIOR TO TAXIING TO THE GATE. UPON ARR; I SET THE PARKING BRAKE; WAITED FOR THE GND PWR SIGN; AND THEN SHUT DOWN ENG #1. THE RAMP AGENT GAVE US THE CHOCKS-IN SIGN AND THE PARKING BRAKE WAS RELEASED. I WAITED A FEW SECONDS AND THEN FOCUSED MY ATTN TO CLEANING UP THE COCKPIT AS WE HAD A CREW CHANGE. ABOUT 15-20 SECONDS LATER I HEARD A POUNDING ON THE SIDE OF THE ACFT AND SHOUTING COMING FROM THE JETWAY. THE CABIN DOOR HAD ALREADY BEEN OPENED AND THE JETWAY BROUGHT TO THE ACFT. AT THAT POINT I NOTICED THE ACFT MOVING BACKWARDS AND SLAMMED ON THE BRAKES. THE ACFT HAD TRAVELED ABOUT 5-8 FT WHERE THE CABIN DOOR CAME IN CONTACT WITH THE JETWAY; PUNCTURING A HOLE IN THE JETWAY; SNAPPING THE CABIN DOOR UPPER GUST LOCK HINGE AND ALSO DAMAGED THE LOWER R INSIDE OF THE CABIN DOOR. THE OUTBOUND FO; WHO WAS CONDUCTING HIS WALKAROUND; INFORMED US THE RAMP AGENTS WERE WALKING AWAY FROM THE ACFT AFTER SHUTDOWN AND THAT THE ACFT DID NOT BEGIN TO MOVE UNTIL 15-20 SECONDS AFTER SHUTDOWN. SUBSEQUENTLY; THE ACFT WAS PULLED FORWARD SO PAX COULD DEPLANE. IT WAS CONFIRMED BY OUTBOUND FO THAT THE CHOCKS WERE NEVER INSERTED. I WOULD HIGHLY RECOMMEND THAT THE COMPANY RECONSIDER ITS NON-USE OF THE PARKING BRAKE WHILE AT THE GATE AFTER SHUTDOWN.